Project Summary/Abstract: Human herpesvirus(HHV)-8-negative, idiopathic multicentric Castleman disease (iMCD) is a deadly hematologic illness involving polyclonal lymphoproliferation and multiple organ system dysfunction. iMCD is diagnosed in approximately 600-1,000 individuals annually in the USA; 35% die within 5 years. Cytotoxic chemotherapies are the only options for the 66% of patients refractory to IL-6 blockade with siltuximab (refractory-iMCD); relapse is common. No new drugs are in development. The etiology,!pathological cell types, and dysregulated signaling pathways are unknown. Improved understanding of disease mechanisms is necessary to identify new treatments. Our preliminary data reveal upregulation of serum vascular endothelial growth factor (VEGF), activated CD8+ T cells, and uncontrolled PI3K/Akt/mTOR signaling in refractory-iMCD patients during flares. Median serum VEGF levels were three-fold above the upper limits of normal in 16 iMCD patients. Proteomic quantification of 315 serum analytes in a refractory-iMCD index case (IC) found that VEGF was the most up-regulated cytokine in flare. A significantly increased fraction of circulating activated HLA-DR+ CD8+ T cells was observed in IC and another refractory-iMCD case compared to controls. Phospho-S6, a read-out of mTOR activity, was dramatically increased in IC and 2/2 other iMCD lymph nodes compared to six reactive and lupus nodes. Moreover, prolonged phosphorylation of Akt was observed in T cell receptor (TCR)-stimulated CD8+ T cells from IC and another refractory-iMCD case. Importantly, administration of the mTOR inhibitor, sirolimus, to IC led to a complete remission lasting five-fold longer than the previous average remission duration. Another patient has had a clinical response lasting two months. We also identified compound heterozygous missense mutations in IC?s CABIN1 gene, a negative regulator of T cell activation, as a potential mechanistic basis for T cell dysregulation in iMCD. We hypothesize that uncontrolled PI3K/Akt/mTOR signaling in activated CD8+ T cells is critical to iMCD pathogenesis, hypersensitivity to TCR-mediated T cell activation is the mechanistic basis, and sirolimus interrupts iMCD by inhibiting mTOR, T cell activation, and VEGF. In Aim 1, we will test whether there is upregulated VEGF, T cell activation, and PI3K/Akt/mTOR signaling in additional refractory-iMCD patients. In Aim 2, we will rigorously evaluate how refractory-iMCD T cells respond to TCR stimulation in vitro. Then, we will test whether the IC?s CABIN1 mutations result in loss-of-function that could predispose to TCR hypersensitivity and search for CABIN1 mutations in more cases. Aim 3 outlines a mechanistic proof of concept study of sirolimus administration to refractory-iMCD patients to investigate PI3K/Akt/mTOR signaling in vivo and document efficacy. The proposed studies will advance our understanding of iMCD by elucidating a novel dysregulated signaling pathway, cell type, cytokine, and genomic alteration in iMCD and may lead to a new treatment paradigm for iMCD and related inflammatory conditions. Using clinical and discovery datasets to uncover a novel use for an existing drug is essential to identify therapies for the 95% of rare diseases with no FDA-approved treatments.